cmd/utils: Log targetGasLimit at startup

[wbt]

Should produce log message like "Current effective targetGasLimit=1234567"
Still working on testing setup.

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[karalabe]

We've merged in #17248 which should cover the original issue prompting this PR too. Now flag's won't silently be interpreted as commands and discarded. I think that's a better solution than individually trying to cover one or more flags.

[wbt]

I think this still could be a useful log message, both for people who don't understand that there is such an option and for people who use the option but keep geth running or store log files for long enough that they lack authoritative reference to the startup command used in that run.